The invention relates to a method of operating an internal combustion engine, more specifically a four-cycle internal combustion engine, that relies for operation, in at least one operational range, on the autoignition of an at least almost homogeneous fuel-air mixture, the combustion being measured and at least one parameter for the combustion in the next cycle being set in function of a signal obtained from this measurement. Furthermore, the invention relates to a device for carrying out this method.
The combustion of an auto-ignited lean fuel-air mixture has the advantage that extremely low NOx and soot emissions are obtained on account of the homogeneous distribution of concentration and temperature. This process is known as HCCI combustion (Homogeneous Charge Compression Ignition). HCCI combustion results in low NOx emissions, which is due to the fact that combustion is initiated at multiple ignition sites, the temperature of the combustion process being relatively low as a result thereof. For HCCI combustion, gasoline presents great advantages over diesel fuel on account of its low autoignition quality and the lower boiling range of between approximately 30xc2x0 C. and 190xc2x0 C. The compression ratio may be raised to values similar to those in a diesel engine of about 15 to 17. Since the precise time of ignition can be fixed as desired just before top dead center only when the effective medium pressure is low, the effective medium pressure achievable in HCCI combustion is disadvantageously limited to the part load range as may be gathered from the publication entitled xe2x80x9cAn Experimental Study on Premixed-Charge Compression Ignition Gasoline Enginexe2x80x9d, Taro Aoyama et al., SAE Paper No. 960081.
DE 199 27 479 A1 describes a method for operating an engine run on gasoline in which the internal combustion engine is operated in the homogeneous charge compression ignition mode when the effective medium pressure is below a predetermined limit and in the spark ignition mode when said effective medium pressure is above said limit. In this way, all the advantages of HCCI combustion can be made use of without the disadvantages thereof.
EP 1 085 192 A2 discloses an internal combustion engine which is operated in the homogeneous charge compression ignition mode in the medium part load range and which is operated in the homogeneous charge spark ignition mode in the upper part load range and at full load, and in the lower part load range as well. In the transition range from the spark ignition mode to the compression ignition mode, the quantity of recirculated exhaust is increased in order to ensure safe autoignition. Conversely, on transitioning from the compression ignition mode to the spark ignition mode of operation, the quantity of recirculated exhaust is reduced in time to prevent knocking.
U.S. Pat. No. 6,230,683 B1 describes a diesel cycle internal combustion engine operating in a homogeneous charge mode in which the combustion process is controlled. Combustion is controlled by controlling the temperature, pressure, autoignition properties and composition of the mixture. During the intake stroke, a gaseous first fuel and a second fuel are supplied to the combustion chamber and, during an early portion of the compression stroke which is to occur between 180 and 60 degrees before top dead center, a pilot fuel is supplied to the combustion chamber. The timing of ignition of said first and second fuel is controlled by the quantity of pilot fuel.
WO 99/40296 A1 discloses a method of operating a four-cycle internal combustion engine run on a homogeneous, lean basic mixture of air and fuel and operated in the compression ignition mode in which the fuel-air ratio generated within the combustion chamber is controlled by a controllable intake element. The respective combustion event is measured and the timing of closing of the intake element is set for the next cycle in function of a signal obtained from said measurement. The engine load is controlled by the timing of closing of the exhaust element and by the residual exhaust gas within the combustion chamber, as well as by the mixture of fuel and unburned gas supplied. Combustion situation and history is sensed in real time by engine parameters such as the structure-borne noise at the internal combustion engine, the ion current in the combustion chamber and the irregularity of rotation of the crankshaft.
Generally, combustion control and torque control are performed together so that compromises must be made. Accordingly, the potential for improvements in consumption and emissions cannot be fully utilized.
Traditional engine controllers for Otto cycle engines operated with a homogeneous charge mode (both Otto cycle internal combustion engines in which injection occurs via intake manifolds and direct-injection Otto cycle internal combustion engines operated in the homogenous charge mode) substantially have the function of providing the three basic parameters, charge, injection and ignition for the respective one of the engine operating points. 8, 16 or 32-bit microcontrollers are generally utilized because they are capable of performing this function. With the help of sensor signals, they detect the actual operating condition of the engine as well as the environmental conditions and determine the optimum parameters mentioned herein above with the assistance of suited characteristic diagrams and curves. In most cases, the characteristic diagrams are addressed through the two basic operating parameters engine speed and engine load. The engine speed is determined by evaluating the signal of a speed sensor that scans a trigger disc mounted on the crankshaft. To ascertain the engine load, three different methods are generally used in principle. The one possibility consists in evaluating the signal of an air mass flow sensor positioned between air filter and throttle that senses the charge of fresh air in the engine as a result thereof. The second method consists in sensing the negative suction pipe pressure prevailing in the collecting suction pipe of the internal combustion engine, the suction pipe pressure being also indicative of the charge of the internal combustion engine. Another method provides the possibility to determine mass flow through the position of the throttle.
These methods serve to sense the load of the internal combustion engine but do not permit to take the pressure and temperature conditions in the cylinder into consideration. However, precisely these two parameters are very important when information about the combustion event or the combustion process is to be given.
Once the charge has been determined, the required mass of fuel to be injected can be determined therefrom, which permits to operate the engine at the desired air-fuel ratio xcex. Deviations from this desired value xcex are sensed by a xcex-sensor located in the exhaust manifold in front of the catalyst and are supplied to a control loop that corrects the deviations from the command value xcex.
Furthermore, actual engine controllers are connected to a pedal valuator that detects the wish of the driver and to an electronic throttle that finally opens to let pass the air mass flow. The attractive power of the pedal valuator onto the throttle is uncoupled, i.e., there is no direct conversion of the pedal value position into a command position of the throttle, the throttle is rather driven by way of a coordination of the gathered torque requirements placed on the internal combustion engine. In this connection, an empiric torque model is mostly used, said model including the inner torque as a primary value. When the timing of ignition is optimally set, the quantity of fresh intake air in an Otto cycle engine operated in the homogeneous charge mode is directly indicative of the torque delivered.
It is the object of the invention to develop a method of operating a four-stroke internal combustion engine of the type mentioned herein above in order to achieve best possible combustion with optimum efficiency and the lowest possible fuel consumption and emissions both in the compression ignition operational range and in the spark ignition operational range.
The solution to this object in accordance with the invention is to rely for operation of the four cycle internal combustion engine in at least one second operational range on the spark ignition of an at least almost homogeneous fuel-air mixture and to control the combustion event, both in the first and in the second operational range of the engine, largely independent of the torque of the internal combustion engine. To carry out the method, the internal combustion engine is provided with a control system with two separate control loops, namely a combustion controller for monitoring the homogeneous combustion on the one side and a torque controller for converting the respective torque requirement on the other side. The combustion event in the first operational range of the engine is thereby controlled by a first fast actuator for the cylinder charge. In the first operational range of the engine the combustion is preferably stabilized on the basis of the evaluation of the combustion event in that the variables controlled are the parameters of the gas exchange valves, preferably the parameters of the injection as well. Parameters for the gas exchange valves are the timing of opening and closing and the lift for intake and exhaust. By contrast, in the first operational range of the engine, the torque is controlled by an actuator for the mass of fuel at least. It is thereby particularly advantageous when, in the first operational range of the engine, the torque is controlled considering the combustion evaluation, the variable used being primarily the mass of fuel, and preferably the parameters of the gas exchange valves as well.
For the control in the second operational range of the engine, the torque is controlled by a first and/or a second slow actuator for the cylinder charge, the first, fast actuator being preferably realized by at least one gas exchange valve and the second slow actuator preferably by an electric throttle. The second, slow actuator is particularly important at speeds near idle when the charge can no longer be correspondingly increased by way of the valve timing. In the spark ignition mode, the control at low speeds near idle thus occurs by way of the electric throttle whereas, at higher speeds in the upper part load range and at full load, the torque is at least partially controlled by the opening and/or closing of at least one gas exchange valve.
In the homogeneous spark ignition mode, the combustion control is carried out by varying the mass of fuel and/or the ignition timing.
To sense the combustion activity there is provided that the combustion is carried out by evaluating a cylinder pressure signal, an ion current signal or the signal of a light-sensitive measuring element.
The first operational range is assigned to the lower part load. The second operational range is, by contrast, assigned to the upper part load and to full load.